Semiconductor mask blanks with a compatible stop layer

ABSTRACT

Provided is a method for creating a mask blank that include a stop layer. The stop layer is optically compatible and process compatible with other layers included as part of the mask blanks. Such blanks may include EUV, phase-shifting, or OMOG masks. The stop layer includes molybdenum, silicon, and nitride in a proportion that allows for compatibility and aids in detection by a residual gas analyzer. Provided is also a method for the patterning of mask blanks with a stop layer, particularly the method for removing semi-transparent residue defects that may occur due to problems in prior mask creation steps. The method involves the detect of included materials with a residual gas analyzer. Provided is also a mask blank structure which incorporates the compatible stop layer.

BACKGROUND

The semiconductor integrated circuit industry has experienced rapidgrowth in the past several decades. Technological advances insemiconductor materials and design have produced increasingly smallerand more complex circuits. These material and design advances have beenmade possible as the technologies related to processing andmanufacturing have also undergone technical advances. In the course ofsemiconductor evolution, the number of interconnected devices per unitof area has increased as the size of the smallest component that can bereliably created has decreased.

Semiconductor fabrication relies heavily on the process ofphotolithography, in which light of a given frequency is used totransfer a desired pattern onto a wafer undergoing semiconductorprocessing. To transfer the pattern onto the wafer, a photomask (alsoreferred to as a mask or reticle) is often used. The photomask permitsand prevents light in a desired pattern onto a layer of the wafer, suchas a photoresist (PR) layer, which chemically reacts to the lightexposure, removing some portions of the PR and leaving other portions.The remaining PR is then used to pattern an underlying layer. As featuresizes have decreased, the wavelength of light used in photolithographyto pattern layers has decreased as well, creating additionaldifficulties and necessitating technological advances such as the use ofextreme ultraviolet (EUV) as a light source and also phase-shiftingmasks. Improving photomasks is important to the continued advances inthe industry, because various kinds of imperfections or problems in theresulting patterned layer can be compounded during subsequent processingsteps as semiconductor device or integrated circuit is made. Photomaskimprovements include improvements made to the mask blank on which thephotomask is derived and patterning the photomask.

For example, during phase-shifting mask fabrication, defects such asthin semi-transparent residue can be formed which often need localizedetching to repair. The localized etching process can result in damage toan underlying mask substrate because a proper end point of the etch maybe difficult to detect. Therefore, while some current techniques forcreating photomasks have been adequate, they have not been entirelysatisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features of the figures are not drawn to scale. Infact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is a flowchart depicting a method for creating an improvedsemiconductor mask blank with an etch stop layer that is opticallycompatible with the other layers.

FIGS. 2A-B depict two diagrammatic fragmentary cross-sectional sideviews of two semiconductor mask blanks which incorporate an opticallycompatible and process compatible etch stop layer.

FIG. 3 is a flowchart depicting a method for patterning an improvedsemiconductor mask with an optically compatible and process compatibleetch stop layer.

FIGS. 4A-F depict a series of diagrammatic fragmentary cross-sectionalside views of a portion of a phase-shifting semiconductor mask blank ina process for patterning the mask for use in semiconductor devicefabrication.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments and examples for implementing different featuresof the invention. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the formation of a first feature over or on a second feature in thedescription that follows may include embodiments in which the first andsecond features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. Various features in the figures may bearbitrarily drawn in different scales for the sake of simplicity andclarity. Various ranges presented may be understood as approximationsgiven the natural variations experiences in semiconductor manufacturingprocesses.

FIG. 1 illustrates a method 100 used for manufacturing a mask blank fora photomask according to various aspects of the present disclosure. Themethod 100 begins with step 102 by receiving a mask blank substrate in amask processing system. The system may include many different tools ormachines, and be capable of many different kinds of processes, such aschemo-mechanical planarization (CMP), material deposition, materialremoval, and physical characterization. The blank mask substrate may beof a variety of suitable materials, including quartz, glass, low thermalexpansion materials (LTEMs), and other materials that may be apparent tothose skilled in the art.

After the mask blank substrate has been received, the method 100continues in step 104 with the deposition of a process-compatible andoptically-compatible stop layer over the substrate. The stop layer isdescribed as process-compatible as it may be made of substantiallysimilar materials and by substantially similar processes as one or moreother layers that may be included on the mask substrate in method 100.Further, the stop layer is described as being optically compatible as itmay be made of materials such that the optical characteristics of thelayer, e.g. reflectivity, dielectric constant, and index of refraction,are similar to those of one or more other layers deposited on the masksubstrate in method 100. Various embodiments of these layers will bediscussed below.

In step 106, a shifter layer is deposited over the blank substrate. Incertain embodiments, the shifter layer may comprise a single effectivelayer comprising molybdenum, silicon, and a nitride such as oxynitride.The shifter layer may serve as a phase-shifting layer, such as may befound in a phase-shifting mask (PSM). Alternatively, the shifter layermay comprise alternating layers of molybdenum and silicon such as areused as the multilayer (ML) portion of a semiconductor mask for use inan extreme ultraviolet (EUV) lithography process. That is, in someembodiments, the shifter layer is situated in between the stop layer andthe substrate. In other embodiments the stop layer is situated inbetween the shifter layer and the substrate.

In step 108, a metal hard mask layer is deposited over the shifterlayer. Materials for the metal hard mask layer may include chromium,chromium oxide, chromium nitride, silicon nitride, silicon oxynitride,tantalum oxide, tantalum nitride, tantalum oxynitride, or other materialsuitable to the reflect or absorb light as required by the process forwhich the mask being formed is intended. The metal hard mask layer maytypically be deposited over the stop and shifter layers. The layersdeposited in various steps of method 100 may be deposited by manydifferent processes. For example, the layers may be deposited bysputtering, physical vapor deposition, chemical vapor deposition, and/orother suitable deposition techniques as may be known in the art.

FIGS. 2A and 2B depict diagrammatic fragmentary cross-sectional sideviews of some embodiments of mask blanks as fabricated according tomethod 100. FIG. 2A depicts a stack of materials suitable for aphase-shifting mask blank 200A. PSM blank 200A includes a quartzsubstrate 202A, a stop layer 204A, a shifter layer 206A, and a hardmetal mask layer 208A. Each layer may underlie the next as listed. In atleast one embodiment, the stop layer 204A may comprise molybdenum,silicon, and oxynitride in a certain proportion, while shifter layer206A comprises molybdenum, silicon, and oxynitride in a differentproportion. In one embodiment, the stop layer 204A includes about 1 to2% molybdenum, 50 to 55% silicon, and 40 to 50% oxynitride by weight;the shifter layer 206A includes about 4 to 5% molybdenum, 45 to 50%silicon, and 40 to 50% oxynitride by weight. The stop layer 204A is 20to 50 Å thick, while the shifter layer 206A is 600 to 700 Å thick. Givensuch layer compositions, the stop layer 204A and the shifter layer 206Amay be produced in the same process step or tool by using an appropriaterecipe to control the composition as desired. The metal hard mask layer208A may be deposited over layers 204A and 206A and may be made ofchromium or a chromium oxide suitable for the requirements of PSM blank200A. Hard metal mask layer may be from 400 to 600 Å thick.

In another embodiment, the stop layer and shifter layer are selectedwith reference to optical properties. For example, the stop layer has areflectivity of less than 15%, an index of refraction between 2.2 and2.4 and a dielectric constant of 0.55 to 0.6; and the shifter layer 206Amay have a reflectively of less than 15%, an index of refraction of 2.5to 2.6, and a dielectric constant from 0.6 to 0.65. Such range values aslisted above should not be construed narrowly, but are approximations ofpotential embodiments of the invention due to inevitable processvariations. The optical properties of the layers in a mask blank areimportant since they affect the physical qualities of all layerspatterned using the masks made from the blanks. Thus, the stop layer204A and the shifter layer 206A have compatible optical characteristicsto limit potential optical problems. While not depicted in FIG. 2A, thephase-shifting mask blank 200A may also comprise a layer of a typical,chemically-amplified resist, e.g. photoresist (PR), about 1500 to 2500 Åthick.

FIG. 2B depicts a stack of materials forming an EUV mask blank 200B. Asits base, the EUV mask blank 200B may have a low thermal expansionmaterial (LTEM) for a substrate 202B. A shifter layer 204B may be aplurality of material layers, alternating between primarily molybdenumlayers and primarily silicon layers. This arrangement is often referredto as a multilayer (ML) stack. So as depicted in FIG. 2B, the shifterlayer 204B is a multilayer stack in some embodiments. On top of theshifter layer 204B, there may be a stop layer 206B. The stop layer 206Bmay be made from a combination of molybdenum, silicon, and oxynitride.In some EUV mask blanks, ruthenium is used as a capping layer on top ofthe ML stack. In some embodiments of the invention, the stop layer 206Breplaces the ruthenium capping layer. This may allow for an EUV maskwith lower reflectivity loss during EUV lithography. The stop layer 206Bmay serve as an effective etch stop for a process, such as anti-chlorideplasma etching, that is normally used to etch an overlying absorberlayer. Additionally, the stop layer 206B may be made in the same processas the ML stack of the shifter layer 204B. Thus, there may besignificant advantages to replacing the ruthenium capping layer with thestop layer 204B made from molybdenum, silicon, and oxynitride. The EUVmask blank 200B may further include an absorber layer 208B, which may bea TaN, silicon nitride, chromium nitride, or other material layersuitable to function as an absorber layer. Above the absorber layer208B, the EUV mask blank 200B may include an anti-reflective coating(ARC) layer 210B. The ARC layer 210B may be made of TaON or othersuitable material.

FIG. 3 illustrates a method 300 for patterning a mask blank, such as aphase-shifting mask (PSM) blank 200A discussed above with reference toFIG. 2A. In step 302, the method 300 begins by receiving a PSM blankinto a mask patterning system, which may include one or more tools usedfor patterning PSM blanks into phase-shifting masks. In step 304, ametal film layer of the mask blank is patterned, such as by using afirst etch process. When a mask blank is patterned, the blank may firstreceive a photoresist (PR) coat or layer. Alternatively, if the blank isreceived with a layer of PR already on it, it may be unnecessary toapply PR at this point in method 300. The layer of PR on top of themetal film layer may be patterned by an electron beam (e-beam) processor other suitable patterning process. After the PR has been patterned,exposed portions of the metal film layer may be removed. This may bedone using an anisotropic dry etch such as a plasma etch or otherappropriate etch.

In step 306, the shifter layer is patterned using a second etchingprocess. By way of example, in step 306 a shifter layer of molybdenum,silicon, and oxynitride is be patterned using a dry etch with SF₆ andO₂. In some embodiments, the shifter layer is patterned using agas-assisted etch in step 306. The metal film layer may serve as theetch mask. During steps 304 and 306, problems may occur such as aparticle dropping onto the mask or other similar problem. These mayresult in a semi-transparent residue of the shifter layer material beingleft behind after the etch designed to remove the shifter layer materialfrom desired locations is mostly complete elsewhere on the mask. Theseresidue defects are often hard to detect, but may still cause undesiredeffects on semiconductor devices fabricated using the resultingphase-shifting mask. Thus the removal of such defects is important toproducing an high-quality mask. While the etch rate of the stop layermay be lower than the etch rate of the shifter layer in this process,some or all of a thickness of stop layer in a given location may beremoved by the etching process in step 306.

In step 308 one or more residue defects left behind after steps 304 and306 may be removed. In some embodiments, the residue comprises only stoplayer material, while in other embodiments it comprises stop layer andshifter layer material. The residue is removed from a small, localizedarea by a localized process, such as gas-assisted etching using ane-beam or focused ion beam tool. The gas used in this etching processmay be XeF₂. This process allows material to be removed from the smallarea of the residue, while avoiding damage to the exposed substrate insurrounding areas. The stop layer is etched by the localized process ata slower rate than the shifter layer. This may help prevent etchingthrough and causing damage to the underlying surface by exposure to thegas-assisted etching.

Additionally, in some embodiments of the invention, a residual gasanalyzer (RGA) is used in conjunction with the gas-assisted etchingtool. The residual gas analyzer is able to detect faint chemical signalsoriginating at the site of localized etching. In the absence of the stoplayer as disclosed herein, determining when to stop the localized etchprocess intended to remove a residue defect may pose particularproblems. These problems may be alleviated or eliminated by the use ofthe stop layer as disclosed. For example, even using a residual gasanalyzer on a phase-shifting mask, without the stop layer, to removedefects can not ensure a proper etch stop point, because as the amountof residue decreases, the signal detected by the residual gas analyzermay become too small to be used reliably. Thus the process of locallyetching residue defects, without the stop layer, may depend heavily on atrial-and-error process and on operator experience. Monitoring theprocess may require repeated removal of the mask from the machine forvisual inspection using various forms of microscopy, riskingcontamination of the mask in handling and delaying production time ineffort to avoid damaging and then scrapping the mask.

With the stop layer present, the process may be improved. For example,using the RGA to detect important points in the etching process may bemore reliable with the stop layer present. In some embodiments of theinvention the stop layer is composed of about 1-2% molybdenum, 50-55%silicon, and 40-50% oxynitride by weight, and the shifter layer iscomposed of about 4-5% molybdenum, 45-50% silicon, and 40-50% oxynitrideby weight. The RGA detects useful chemical signals from such acombination, when it could not from the shifter layer alone. First, theRGA may detect the transition from shifter layer material to stop layermaterial as it is being removed from the localized area of the residuedefect. This may provide a clear indication of the amount of materialthat remains to be removed in order to remove the entire residue defect.Second, the RGA may more easily detect a chemical signal that indicatesthat the stop layer material has been removed. Thus, embodiments of theinvention may provide clearer chemical signals indicating the removal ofthe shifter layer and the removal of the stop layer. Using these clearerchemical signals, the process of removing residue defects may besimplified and systematized in a way that does not depend so heavily onoperator experience or a trial-and-error approach.

For example, while undergoing the localized etch process to remove anidentified residue defect, a user of the method and masks as disclosedherein detects a first chemical signal which indicates that thelocalized etch process has successfully removed the shifter layermaterial from the localized area and that stop layer material is nowbeing etched. A time later, a second chemical signal indicates that thestop layer material of the residue defect has been removed from thelocalized area and that the etch process is complete. The first chemicalsignal may be associated with the compositional differences between theshifter and stop layers. The second chemical signal may be associatedwith the exhaustion of stop layer material from the location, and thusrelated to the presence or absence of the stop layer material. Step 308of method 300 ends when the stop layer material of the defect is removedand that removal is detected.

FIGS. 4A-F illustrates a series of diagrammatic fragmentarycross-sectional side views of a portion of a phase-shifting mask (PSM)blank 400 in a process for patterning a mask for use in semiconductordevice fabrication. FIGS. 4A-F depict a portion of the PSM blank 400 atvarious mask fabrication stages according to embodiments of the presentdisclosure.

FIG. 4A depicts the PSM blank 400 before any processing steps have beenundertaken. The PSM blank 400 includes a substrate 402, a stop layer404, a shifter layer 406, and a metal hard mask 408. These layers may becomprised of various materials as indicated elsewhere in thisapplication. In at least one embodiment the layers of FIG. 4A are thesame as those of FIG. 2A, and this embodiment will be used to provideillustrative detail to FIGS. 4A-F. Thus, the substrate 402 is a quartzsubstrate, the stop layer 404 comprises molybdenum, silicon, andoxynitride combined in a first proportion, the shifter layer 406comprises molybdenum, silicon, and oxynitride combined in a secondproportion different from the first, and the metal hard mask 408 ischromium.

FIG. 4B depicts the PSM blank 400 with a patterned PR layer 410 over it.The patterned PR layer 410 has been deposited earlier and patterned bye-beam lithography before being developed as a masking layer forpatterning the metal hard mask 408. The geometry of the patterned PRlayer 410 is designed to create an intended opening which will exposethe substrate 402. PSM blank 400 is exposed to a first etch process 420suitable for removing the exposed chromium metal hard mask 408.

The result of the first etch process 420 is depicted in FIG. 4C. Theportion of the metal hard mask 408 that was exposed has been removed.While, not otherwise depicted, the patterned PR layer 410 has also beenremoved from the rest of the metal hard mask 408. The PSM blank 400 asdepicted in FIG. 4C is then subjected to a second etch process 422.

The result of the second etch process 422 is depicted in FIG. 4D. Whilenot always the case, FIG. 4D depicts the second etch process 422 ashaving resulted in a residue defect 412. As shown in FIG. 4D, theresidue defect 412 includes portions of the shifter layer 406 and thestop layer 404. In some other embodiments residue defect 412 onlyincludes portions of the stop layer 404. After an inspection processindicates the presence and location of the residue defect 412, a firstlocalized etch process 424 is applied to an area limited to the residuedefect 412 or a portion thereof.

The result of the first localized etching process 424 may be depicted inFIG. 4E. The portion of the shifter layer 406 that formed part of theresidue defect 412 has been removed. This may result in a first chemicalsignal being detected by an RGA in communication with the etchingchamber. The residue defect 412 is subjected to a second localized etchprocess 426. In at least one embodiment, second localized etch process426 is a mere continuation of the first localized etch process 424 andnot a discrete, separate etch process.

The result of the second localized etch process 426 is depicted in FIG.4F. The stop layer 402 portion of the residue defect 412 has beenremoved, thus the entire residue defect 412 has been removed from thelocalized area, and the intended opening determined by the geometry ofthe patterned PR layer 410 (as seen in FIG. 4B) has been accuratelyformed. As the last small amounts of the stop layer 404 portion ofresidue defect 412 are removed, a second chemical signal may be detectedby the RGA. This second chemical signal indicates that the desiredresult of the second etch process 422 has been achieved, and that thesecond localized etch process 426 should be stopped to avoid damage tothe portion of the substrate 402 exposed by the removal of the residuedefect 412.

Throughout FIGS. 4A-F, it should be apparent to those skilled in the artthat some additional steps may occur. For example, some developing,stripping, cleaning, and exposure steps have not been portrayed orexplained. This has been done to simplify the disclosure and clarifyvarious embodiments of the invention. It should be understood that suchprocesses occur where necessary or advantageous.

The embodiments discussed above offer advantages over conventionalphase-shifting and EUV masks. The use of the stop layer as described andenvisioned by this disclosure provides a mechanism for recognizingimportant points in the process of removing residue defects. Byrecognizing those important points in a consistent manner, and notrelying on operator experience, damage to the underlying mask can beavoided. Further the stop layer may provide for a material that isoptically compatible with an EUV mask and a phase-shifting mask and isalso compatible with the fabrication processes and materials used tocreate other aspects of EUV and phase-shifting masks, such as the MLstack and the phase shifter respectively. Thus the beneficial stop layermay not require more process steps or tools than currently used and maynecessitate fewer. For example, the stop layer as used in the EUV maskreplaces the ruthenium layer and its associated processing steps.

One embodiment of the invention is a method for creating aphase-shifting semiconductor mask blank. The method comprises steps ofreceiving a cleaned and polished mask substrate, depositing a stop layerover the mask substrate, depositing a shifter layer over the stop layer,and depositing a metal hard mask layer over the shifter layer. In atleast some embodiments depositing the stop layer and the shifter layeris performed in a single process. In a particular embodiment, the stoplayer comprises molybdenum, silicon, and oxynitride in a firstproportion; and the shifter layer comprises molybdenum, silicon, andoxynitride in a second proportion. The first proportion of molybdenum,silicon, and oxynitride are not equal to the second proportion.

In other embodiments, the shifter layer is comprised of a materialhaving a reflectivity less than 15%, an index of refraction ranging from2.2 to 2.4, and a dielectric constant ranging from 0.55 to 0.6; and thestop layer is comprised of a material having a reflectivity less than15%, an index of refraction ranging from 2.5 to 2.6, and a dielectricconstant ranging from 0.6 to 0.65.

Another embodiment of the invention includes a method for removingresidue defects from a phase-shifting semiconductor mask blank in aprocess to create a phase-shifting mask. The method may comprise stepsof receiving a PSM blank, etching a metal film layer of the PSM blank,and etching an exposed portion of a shifter layer of the PSM blank. Themethod further includes etching a localized area of the exposed portionto remove a residue defect. The residue defect comprises a portion ofthe shifter layer and a portion of a stop layer underlying the shifterlayer. In certain embodiments, etching the localized area of the exposedportion of the shifter layer to remove the residue defect is performedusing a localized etching process such as, but not necessarily limitedto, gas-assisted etching with an e-beam or focused ion beam tool.

In yet another embodiment, the method includes detecting a firstchemical signal, which indicates the removal of the shifter layerportion of the residue defect from the localized area. The methodfurther includes detecting a second chemical signal, which indicates thestop layer portion of the residue defect has been removed from thelocalized area. The method comprises using at least the first chemicalsignal to determine when to stop the localized etching process. Themethod also comprises using the second signal to determine when to stopthe associated etch. The stop layer comprises molybdenum, silicon, andoxynitride combined in a first proportion, and the shifter layercomprises molybdenum, silicon, and oxynitride combined in a secondproportion.

Another embodiment of the invention is a mask blank. The blank comprisesa transparent substrate, a stop layer over the substrate formed from aset of materials combined in a first proportion, a shifter layer overthe substrate formed from the set of materials combined in a secondproportion, and an absorber layer. The first proportion of the set ofmaterials causes the stop layer to have a first etch rate, while thesecond proportion of the set of materials causes the shifter layer tohave a second etch rate, the second etch rate being higher than thefirst etch rate. The set of materials comprises silicon, molybdenum, andoxynitride. The stop layer and the shifter layer are made by the samedeposition process.

In a given embodiment, the first proportion comprises molybdenum in arange from 4 to 5 percent, silicon in a range from 45 to 50 percent, andoxynitride in a range from 40 to 50 percent; and the second proportioncomprises molybdenum in a range from 1 to 2 percent, silicon in a rangefrom 50 to 55 percent, and oxynitride in a range from 40 to 50 percent.The first proportion causes the stop layer, when undergoing an etchingprocess, to be more detectable by a residual gas analyzer than theshifter layer, when also undergoing the etching process. Additionally,the reflectivity of the stop layer is approximately equal to thereflectivity of the shifter layer.

In another embodiment, the disclosed features of the invention areadapted to an opaque MoSi on glass (OMOG) mask.

In a particular embodiment of the invention, a mask blank comprises asubstrate, a shifter layer which includes a plurality of molybdenum andsilicon multilayer structures, a stop layer which is situated over theplurality of multilayer structures, and is comprised of materialsincluding molybdenum and silicon. The substrate is an LTEM substrate,and the absorber layer is an EUV absorber layer; further comprising ananti-reflective coating over the EUV absorber layer. In such anembodiment, the stop layer comprises molybdenum in a range from 1 to 2percent; silicon in a range from 50 to 55 percent; and oxynitride in arange from 40 to 50 percent.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

1. A method for creating a phase-shifting semiconductor mask blank, themethod comprising: receiving a cleaned and polished mask substrate;depositing a stop layer over the mask substrate; depositing a shifterlayer over the stop layer; and depositing a metal hard mask layer overthe shifter layer.
 2. The method of claim 1, wherein depositing the stoplayer and the shifter layer is performed in a single process.
 3. Themethod of claim 2, wherein: the stop layer comprises molybdenum,silicon, and oxynitride in a first proportion; the shifter layercomprises molybdenum, silicon, and oxynitride in a second proportion;and the first proportion is not equal to the second proportion.
 4. Themethod of claim 2, wherein: the shifter layer is comprised of a materialhaving a reflectivity less than 15%, an index of refraction ranging fromabout 2.2 to 2.4, and a dielectric constant ranging from about 0.55 to0.6; and the stop layer is comprised of a material having a reflectivityless than 15%, an index of refraction ranging from about 2.5 to 2.6, anda dielectric constant ranging from about 0.6 to 0.65.
 5. A method forcreating a phase-shifting mask, the method comprising: receiving aphase-shifting mask blank, the phase-shifting mask blank comprising ametal film layer, a shifter layer, and a stop layer; etching the metalfilm layer of the phase-shifting mask blank; etching an exposed portionof the shifter layer of the phase-shifting mask blank; etching alocalized area of the exposed portion to remove a residue defect, theresidue defect comprising a portion of the shifter layer and a portionof the stop layer underlying the shifter layer.
 6. The method of claim5, wherein etching the localized area of the exposed portion of theshifter layer to remove the residue defect is performed using agas-assisted etch with an e-beam tool or a focused ion beam tool.
 7. Themethod of claim 6, further comprising detecting a first chemical signal,the first chemical signal indicating the removal of the portion of theshifter layer of the residue defect from the localized area.
 8. Themethod of claim 7, further comprising: detecting a second chemicalsignal, the second chemical signal indicating the portion of the stoplayer has been removed from the localized area.
 9. The method of claim8, further comprising: using at least the first chemical signal todetermine when to stop the localized etching process.
 10. The method ofclaim 5, wherein: the stop layer comprises molybdenum, silicon, andoxynitride combined in a first proportion; the shifter layer comprisesmolybdenum, silicon, and oxynitride combined in a second proportion; andthe first proportion is different from the second proportion.
 11. Asemiconductor mask, the mask comprising: a transparent substrate; a stoplayer over the substrate, the stop layer being formed from a set ofmaterials combined in a first proportion; a shifter layer over thesubstrate, the shifter layer being formed from the set of materialscombined in a second proportion; and an absorber layer.
 12. The mask ofclaim 11, wherein: the first proportion of the set of materials causesthe stop layer to have a first etch rate; and the second proportion ofthe set of materials causes the shifter layer to have a second etchrate, the second etch rate being higher than the first etch rate. 13.The mask of claim 11, wherein the set of materials comprises silicon,molybdenum, and oxynitride.
 14. The mask of claim 13, wherein the firstproportion comprises: molybdenum in a range from about 4 to 5 percent,silicon in a range from about 45 to 50 percent, and oxynitride in arange from about 40 to 50 percent; and the second proportion comprises:molybdenum in a range from about 1 to 2 percent, silicon in a range fromabout 50 to 55 percent, and oxynitride in a range from about 40 to 50percent.
 15. The mask of claim 11, wherein the stop layer, whenundergoing an etching process, is more detectable by a residual gasanalyzer than the shifter layer, when undergoing the etching process.16. The mask of claim 11, wherein a reflectivity of the stop layer isequal to a reflectivity of the shifter layer.
 17. The mask of claim 11,wherein the stop layer and the shifter layer are made by the sameprocess.
 18. The mask of claim 11, wherein at least a portion of thestop layer has been removed by a localized etching process to remove aresidue defect, the residue defect comprising the portion of the stoplayer, the localized etching process having been terminated by detectinga chemical signal indicating the removal of the portion of the stoplayer.
 19. The mask of claim 11, wherein: the shifter layer comprises aplurality of molybdenum and silicon multilayer structures; the stoplayer is situated over the plurality of multilayer structures andcomprises materials including molybdenum and silicon; the substrate isan LTEM substrate; and the absorber layer is an EUV absorber layer; andfurther comprising an anti-reflective coating over the EUV absorberlayer.
 20. The mask of claim 19, wherein the stop layer comprises:molybdenum in a range from about 1 to 2 percent; silicon in a range fromabout 50 to 55 percent; and oxynitride in a range from about 40 to 50percent.